The present invention relates to a housing fixing structure, in particular, to a housing fixing structure for fixing a housing of an electronic control device which performs a safety control based on a signal of an impact detection sensor.
FIG. 1 is a schematic view illustrating an attachment position of an airbag electronic control device (hereinafter, referred to as “airbag ECU”) 70 according to a related art. FIG. 2 is a perspective view of the constituent components of the airbag ECU 70 when viewed obliquely from the front and right side. As indicated by arrows in FIG. 1, hereinafter, the front-back direction, the left-right direction, and the up-down direction are defined with reference to a vehicle on which the airbag ECU 70 is to be mounted. That is, the front-back direction is the longitudinal direction of the vehicle, the left-right direction is the width direction of the vehicle, and the up-down direction is the height direction of the vehicle. FIG. 3 is a schematic view illustrating a board 72 when viewed from directly above according to a related art.
A support surface 73 supporting the airbag ECU 70 is disposed below an instrument panel 71 provided at the front central portion inside the vehicle. The airbag ECU 70 is screwed to the support surface 73 at three points to be fixed to the vehicle. A connector 74 is provided at the rear portion of the airbag ECU 70, and a connection portion of the connector 74 is provided at an opening formed at the rear side surface of a housing 75. An attachment bracket 76a for fixing the airbag ECU 70 to the support surface 73 of the vehicle is provided at the right side of the rear portion of the airbag ECU 70. Attachment brackets 76b and 76c for fixing the airbag ECU 70 to the vehicle are respectively provided at the left and right sides of the front portion of the airbag ECU 70. No attachment bracket is provided at the left side of the connection portion of the connector 74 (at the left side of the rear portion of the airbag ECU 70).
As shown in FIG. 3, an acceleration sensor 77 for detecting an impact of a vehicle collision and the connector 74 are mounted on the board 72. As shown in FIG. 2, the board 72 on which the acceleration sensor 77 and the connector 74 are mounted is screwed to the housing 75 from below the housing 75 together with a cover 78 provided below the board 72.
FIG. 4 is a schematic view illustrating the airbag ECU 70 according to the related art when viewed from below. FIG. 5 is an enlarged view illustrating the attachment bracket 76a disposed at the rear right side of the connector 74 when viewed from below. The impact applied to the vehicle is transferred to the attachment brackets 76a, 76b, and 76c of the housing 75 and then transferred from the attachment brackets 76a, 76b, and 76c to the board attachment screw bearing surfaces (not shown) of the housing 75. The impact transferred to the board attachment screw bearing surfaces is transferred to the board 72 via board attachment screws 79. The acceleration sensor 77 mounted on the board 72 detects the impact transferred to the board 72 and then outputs a detection signal to the airbag ECU 70. The airbag ECU 70 outputs an ignition signal for deploying the airbag when it is determined that the degree of the impact of the vehicle collision exceeds a predetermined threshold value based on the detection signal.
When excessive impact is applied to the vehicle due to the vehicle collision, the attachment bracket 76a disposed at the rear portion of the housing 75 so as to fix the airbag ECU 70 to the support surface 73 is broken, in order to prevent the board 72 from being deformed or broken. With the attachment bracket 76a being broken, the impact transferred from the vehicle to the board 72 is reduced. Accordingly, when the excessive impact is applied to the vehicle, the attachment bracket 76a has to be broken. In addition, the bracket portion needs to be further reduced in weight. Therefore, the attachment bracket 76a is molded such that the width T2 of a connecting portion connecting a fixing portion 80 of the attachment bracket 76a and the housing main body is narrow within the range without causing any problem involving strength.
Patent Document 1 discloses a housing structure in which an impact transferred to a housing is well transferred to a board accommodated in the housing, and deformation of the board can be suppressed on the basis of distortion in the housing. Patent Document 2 discloses a configuration in which a short protrusion is formed in order to protect a casing from an impact due to falling when the casing falls down. The protrusion is made of the same material as the casing and can be formed integrally with the casing. Patent Document 3 discloses a configuration in which vibration from a single printed board on which a processing circuit is mounted is unlikely to be transferred to an acceleration sensor even if the acceleration sensor and constituent components of the processing circuit are mounted on the single printed board.
Patent Document 4 discloses a configuration in which a sensor is fixed to a casing to prevent motions other than motions in the casing from being transferred to the sensor in order to prevent vibration in the up-down direction of a vehicle from being erroneously detected as vibration in the longitudinal direction of the vehicle when the sensor assembly is mounted on the vehicle.
Patent Document 5 discloses a configuration in which a casing of a collision detection device is composed of a cover and a housing which are fitted with each other in order to prevent the collision detection device from being easily disassembled.
Patent Document 6 discloses a collision detection device in which an acceleration sensor and a microcomputer determining collision are provided in a unit case, such that vibration of the unit case and constituent components is unlikely to be transferred to the acceleration sensor.    Patent Document 1: JP-A-2005-155827    Patent Document 2: JP-T-2000-517474    Patent Document 3: JP-A-9-207706    Patent Document 4: JP-A-9-159689    Patent Document 5: JP-A-8-40185    Patent Document 6: JP-A-7-196008
FIG. 6 is a graph illustrating a measurement result of the resonance characteristics of the airbag ECU 70 according to the related art in a low frequency region. Vibration is applied to the airbag ECU 70 in the front-back direction, the left-right direction, and the up-down direction of the vehicle, and the resonance generated in the respective directions are measured. X1 represents the resonance in the front-back direction (the traveling direction) of the vehicle, Y1 represents the resonance in the left-right direction of the vehicle, and Z1 represents the resonance in the up-down direction of the vehicle. A dashed-two dotted line A represents boundary of an NG region where it is determined that the vehicle collision occurs. According to the resonance characteristics shown in FIG. 6, the resonance in the left-right direction overlapping the line A is generated. When the resonance which enters the NG region is generated, the acceleration sensor 77 may detect an impact which should not be detected. Although the connector 74 is the heaviest component mounted on the board 72, the connector 74 is not securely fixed to the vehicle since no attachment bracket is provided at the left rear side of the connector 74. That is, there is a lot of flexibility at the left rear side of the connector 74. When the left side of the connector 74 swings greatly, the resonance which enters the NG region is generated. However, the Patent Documents 1 to 6 fails to describe a configuration in which the resonance of the board in the low frequency region can be effectively suppressed.